Add Checkstyle checking during Maven build (#138)

So far no rules are activated and it just puts the infrastructure in place.
Rules can be added in separately after discussion among the developers,
along with fixing any violations of the rules.

Co-authored-by: Nico Athanassiadis <nico@dsv.su.se>
Reviewed-on: #138
Reviewed-by: Nico Athanassiadis <nico@dsv.su.se>
Co-authored-by: Andreas Svanberg <andreass@dsv.su.se>
Co-committed-by: Andreas Svanberg <andreass@dsv.su.se>

Dockerfile

@@ -24,6 +24,7 @@ COPY test-data/src/ test-data/src/
 RUN ./mvnw package \
     --define skipTests \
     --activate-profiles branch,DEV \
+    --define checkstyle.skip=true \
     --define skip.npm \
     --define skip.installnodenpm
 

checkstyle.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+        "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<module name="Checker">
+</module>

pom.xml

@@ -211,6 +211,11 @@
                     <artifactId>maven-war-plugin</artifactId>
                     <version>3.4.0</version>
                 </plugin>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-checkstyle-plugin</artifactId>
+                    <version>3.6.0</version>
+                </plugin>
             </plugins>
         </pluginManagement>
         <plugins>
@@ -312,6 +317,23 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-checkstyle-plugin</artifactId>
+                <configuration>
+                    <configLocation>checkstyle.xml</configLocation>
+                    <failOnViolation>true</failOnViolation>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>validate</id>
+                        <phase>process-sources</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
         </plugins>
     </build>
 </project>